Seraphine Hauser

Weather and Climate Systems 

Characteristics of large-scale circulation “forecast busts” over Europe in ERA5 (re-)forecasts 

Seraphine Hauser

Wednesday, May 8th

NWC 1350 / 3:00 pm

Abstract: Despite rapid improvement in global-scale numerical weather prediction systems, forecast centers repeatedly experience notable declines in their systems’ forecast accuracy. These occasional very poor forecasts are often referred to as “forecast busts” and lead to perceptible differences in the anticipated weather. It is therefore important to understand the causes and the origins of “forecast busts” through a systematic analysis. Previous work (e.g., Rodwell et al., 2013; Lillo and Parsons, 2017) investigated the mean characteristics but also the different flavors of “forecast busts” based on ERA-Interim (re– ) forecasts from ECMWF. However, an analysis of these events in ECMWF’s most recent reanalysis (ERA5), which provides a considerably longer period set, is still missing. In this study, we identify “forecast busts” over Europe based on the day 6 anomaly correlation coefficient (ACC) forecast skill of geopotential height at 500hPa (Z500) using 42 years of ERA5 (1979-2020). Characteristics of these events are compared to the characteristics of “forecast busts” in an earlier model version (ERA-Interim), but also – for the first time – compared to exceptionally good forecasts in order to gain a better understanding of the strengths and weaknesses of the CY41R2 of the ECMWF Integrated Forecast System in predicting the large-scale pattern over Europe. To further extend the range of analyses on “forecast busts”, we systematically investigate various flavors of error patterns over Europe at the day 6 forecast and discuss their distinct error pattern evolution and their weather impacts over Europe.